1. Field of the Invention
The present invention generally relates to a wireless communication system and a handover method and an apparatus for use therein and, and more specifically, to a Wireless Local Area Network (WLAN) system and a handover method and an apparatus for use therein.
2. Description of the Related Art
The current wireless communication system has become a system capable providing data communication services beyond the voice-oriented services. One of the representative wireless communication systems capable of providing high speed data communication services is Wireless Local Area Network (WLAN). The WLAN system is deployed in local area environment such as home, company, and office, and useful in various locations, e.g. a department store, a shopping mall, and an airport.
With the advance of the electronic and mobile communication technologies, the mobile communication terminals are designed to use the WLAN services while receiving the cellular communication service provided by the mobile carrier. The reason for embedding the WLAN function in the mobile communication terminal is that a data communication service such as data download can be provided without extra cost or at a low cost using the WLAN system, and thus is cost-effective as compared to the cellular communication network-based data communication service. Typically, the WLAN-enabled cellular communication terminal is configured to select the WLAN service for data communication with priority.
Meanwhile, the WLAN system provides the user with handover service to guarantee the mobility of the terminal like the most mobile communication systems that provide handover services to the users on the move. The handover function is provided in such a procedure that the terminal changes its connection to the network seamlessly when it moves from one service area to another service area.
In order to secure the terminal mobility even in the WLAN system, a handover procedure has been specified. A description is made briefly of the handover method specified for use in the WLAN system. The terminal (station) connects to an Access Point (AP) having an Extended Service Set (ESS) to receive services. If the received signal strength from the AP is weakened or no beacon signal is received in a predetermined time, the terminal determines that a handover condition is fulfilled. In this case, the terminal attempts handover to another AP.
In the WLAN system, typically, the ESS has one AP with the exception of the intra-company WLAN and some shopping malls. Accordingly, if a station receiving the data service in the ESS having one AP moves out of the service area, its handover attempt is unnecessary because there is no other AP.
Suppose a station capable of receiving data service through a cellular communication network as well as the WLAN system in the above situation. If the station receiving the data service in the ESS having one AP moves out of the service area of the AP, the station attempts handover to another AP. If it fails to discover another AP, the station continues to receive the data service through the cellular communication network. As described above, the station which receives data service in the ESS having only one AP consumes power unnecessarily by attempting the handover of no necessity.
Consider a case where the handover between the WLAN system and the cellular communication network is supported. It is typical that the station is configured to receive the data service through the WLAN system as far as possible rather than the cellular communication network. Accordingly, if the terminal in the state of receiving the data service in the ESS having one AP moves out of the service area of the AP, it attempts handover to another AP without priority before handover to the cellular communication network.
If such unnecessary handover procedure is repeated in the state of receiving the data service, this causes unnecessary power consumption of the station and increases unnecessary time delay for handover to the cellular communication network.